Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with a graphical user interface showing our new design; and,

FIG. 2 is a front view of a second embodiment thereof.

The outermost broken lines in the figures represent the display screen or portion thereof that forms no part of the claimed design. The remaining broken lines in the figures show portions of the graphical user interface that form no part of the claimed design. 

CLAIM I claim the ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 